Crossing time vs. age for an all-sky sample of 236 clusters (Piskunov et al. 2006) taken from an homogeneous catalog of 650 optical clusters in the solar neighborhood (Kharchenko et al. 2005b,a). The dashed line is the identity t_cross = Age, which divides the physical OCs (t_cross ≤ Age) from associations (t_cross>Age) according to the criterion proposed by Gieles & Portegies Zwart (2011).